Wednesday, September 30, 2009

How Sue Died

Photo by mharrsch, via flickr

If you've seen Sue, you may have seen those holes on her jaw (as shown above) and wondered what caused them. While they superficially look like bite marks, they have been attributed to a bacterial infection called Actinomyces. But a new study led by Ewan D. S. Wolff of the University of Wisconsin and published by PLos ONE arrives at a new, intriguing conclusion: The holes are caused by a disease similar to one called Trichomonas which is found in modern birds.

This story has a couple of interesting aspects to me. First, it's another piece of circumstantial evidence linking dinosaurs and birds - the authors believe that the parasite that causes the disease may be an ancestor of the one that infects modern birds. Second, the authors note that it sheds further light into the behavior of the Tyrannosaur clan. From the paper:

Given the ways in which Trichomonas infection is spread among extant birds, the occurrence of a similar disease in tyrannosaurids suggests five possible scenarios for transmission: water-borne transmission, feeding of tainted prey to nestlings, consumption of infected prey, cannibalism, and snout to snout contact during face biting between adults or between infected adults and nestlings.

The authors strike water-borne transmission and tainted baby food straight away, because the fossil record doesn't allow us to explore these options - we've never found a tyrannosaur nest, and even if we did, it's possible that a baby would die well before such large lesions could develop. Consumption of infected prey is eliminated as well, as we've never found evidence of the infection in any of the species tyrannosaurids are known to have preyed upon.

This leaves two methods of transmission observable in the fossil record: snout to snout contact during face-biting and cannibalism. These are both known to be part of tyrannosaur lives; while the bite marks of smaller tyrannosaurids such as Daspletosaurus and Albertosaurus are harder to distinguish, those left by T. rex are unmistakable. And they left them on each other often. They appear to have been aggressive towards one another, using this face-biting behavior to spar over territory, mating rights, carrion or felled prey, or some other contentious aspect of tyrannosaur life. This behavior probably helped spread the disease.

Once infected, the Trichomonas-like parasite would have resided as a film in the tyrannosaur's throat. A badly infected, lesion-bearing individual, as Sue seems to have been, would find swallowing difficult, eventually falling to starvation.

Illustration by Chris Glen of The University of Queensland, via the PLoS ONE paper